1. Field of the Invention
The present disclosure relates to alternating current (AC) light emitting devices and, more particularly, to an AC light emitting device operated by an AC voltage source, a driving device thereof, and a driving method thereby.
2. Discussion of the Background
A highly integrated light emitting diode (LED) chip or package includes a plurality of LED cells connected to each other and generally undergoes an increase of a threshold voltage in proportion to the number of LED cells connected to each other in a forward direction when used with an AC voltage source.
FIGS. 1 and 2 are circuit diagrams explaining problems of a conventional AC LED.
FIG. 1 is an equivalent circuit diagram where an AC LED 10, an AC voltage source Vac, and a resistor R1 are connected in series to one another. In the AC LED 10, an LED cell 12 is operated in a positive half-period of the AC voltage source Vac and an LED cell 14 is operated in a negative half-period of the AC voltage source Vac. The current across the AC LED 10 is dependent on the resistor R1.
FIG. 2 is a graph depicting a voltage-current waveform of the AC LED shown in FIG. 1, in which g1 is a voltage curve of the AC voltage source Vac and g2 is a current curve of the current across the AC LED 10 in FIG. 1. The x-axis indicates time and the y-axis indicates voltage or current.
Referring to FIGS. 1 and 2, in the positive half-period of the AC voltage source Vac, a current is allowed to flow and the LED cell 12 emits light when the voltage is greater than or equal to the forward threshold voltage of the LED cell 12. Similarly, in the negative half-period of the AC voltage source Vac, a current is allowed to flow and the LED cell 14 emits light when the voltage is greater than or equal to the forward threshold voltage of the LED cell 14.
Such characteristics of the AC LED 10 operated only by application of the voltage greater than or equal to the forward threshold voltage cause several problems. In other words, when the applied voltage is greater than or equal to the forward threshold voltage of the LED cell 12 or 14 connected in the forward direction with respect to the applied voltage, an avalanche current flows through the AC LED 10 and a short operating region is provided to the AC LED 10 for a single period of the AC voltage source applied thereto, thereby causing a low power factor, an increase in total harmonic distortion (THD) (particularly, third-order harmonic distortion), excessive flickering, and deterioration in optical efficiency.